A. Field of the Invention
The present invention relates generally to ballasts for driving fluorescent lamps, and more specifically to methods and apparatus for driving fluorescent lamps and to methods and apparatus for coupling ballasts to lamp circuits.
B. Related Art
Many lighting systems, for example those using fluorescent lamps of the T10 and T12 sizes, use traditional iron core and coil ballasts. These types of ballasts are generally large and generate heat resulting in poor power efficiency. Such ballasts also tend to poorly regulate their output, especially with voltage fluctuations, often causing flicker in the light output. Inadequate output regulation resulting in flicker reduces the quality of the illumination provided by the lighting system. Where lighting systems are used in product display cases, the flicker results in a poorer quality illumination and often causes irritation for viewers.
In low temperature applications, such as in refrigerated display cases, conventional ballasts placed inside a freezer, for example, results in increased power consumption. If the ballast is placed outside the freezer, the ballast reliability decreases. Moreover, such conventional ballasts are typically large in size and difficult to package, transport and install in the limited spaces provided by such display cases.
Lighting systems are often cycled on and off, such as between the times when a store is open and when a store is closed. Ordinarily, start-up of a lighting system applies full voltage to the lamp immediately. This full-on condition places a significant load on the cathode of the lamp, especially when the cathode is at the temperature of a refrigerated display case. This temperature may be either a cooler or a freezer, the cooler operating at less than 10.degree. Celsius, and the freezer operating less than -10.degree. Celsius. This heavy load on the cold cathode results in significantly reduced lamp life, and attendant higher costs of operation.
While some ballasts may operate at a higher driving frequency and a higher driving voltage for the lamp, such higher frequency and voltage produce significant heat and require significant heat dissipation from the ballasts. Because of the heat generated, a larger number of failures may occur if parts prematurely wear out.
Most ballasts have limited ranges of application. If a ballast is intended for one line configuration, such as in the United States at 110 volts and 60 Hertz, or according to the European convention of 220 volts at 50 Hertz, different ballasts are presently supplied according to the required convention. Consequently, large inventories are required to accommodate the variations in requirements, and the operation of any given ballast is often affected by variations in line voltage, such as occurs during normal operation. In the refrigeration cases, fluctuations often occur in line voltage due to starting and stopping of compressors and other equipment. Such fluctuations cause lamp flicker or other non-uniform light output with a concurrent effect on the viewer.
These effects resulting from the use of conventional ballasts lead to shorter lamp lives, inefficiencies in operation, and possibly increased ballast failure. Therefore, improvements in ballast design and operation can be made.